Insulin-like growth factors (IGFs) are part of a complex system that cells use to communicate with their physiologic environment. This complex system (often referred to as the insulin-like growth factor axis) consists of two cell-surface receptors (IGF-1R and IGF-2R), two ligands (IGF-1 and IGF-2), a family of six high-affinity IGF-binding proteins (IGFBP 1-6), and associated IGFBP degrading enzymes (proteases). This system is important not only for the regulation of normal physiology but also for a number of pathological states (Glass, Nat Cell Biol 5:87-90, 2003).
The IGF axis has been shown to play roles in the promotion of cell proliferation and the inhibition of cell death (apoptosis). IGF-1 is mainly secreted by the liver as a result of stimulation by human growth hormone (hGH). Almost every cell in the human body is affected by IGF-1, especially cells in muscles, cartilage, bones, liver, kidney, nerves, skin and lungs. In addition to the insulin-like effects, IGF-1 can also regulate cell growth. IGF-1 and IGF-2 are regulated by a family of gene products known as the IGF-binding proteins. These proteins help to modulate IGF action in complex ways that involve both inhibiting IGF action by preventing binding to the IGF receptors as well as promoting IGF action through aiding delivery to the receptors and increasing IGF half life in the blood stream. There are at least six characterized binding proteins (IGFBP 1-6).
In its mature form, human IGF-1 (gpetlcgaelvdalqfvcgdrgfyfnkptgygsssrrapqtgivdeccfrscdlrrlem ycaplkpaksa; SEQ ID NO:1), also called somatomedin, is a small protein of 70 amino acids that has been shown to stimulate growth of a wide range of cells in culture. The mature protein is initially encoded by three known splice variant mRNAs. The open reading frame of each mRNA encodes a precursor protein containing the 70 amino acid IGF-1 and a particular E-peptide at the C-terminus, depending on the particular IGF-1 mRNA. These E-peptides have been termed the Ea (rsvraqrhtdmpktqkevhlknasrgsagnknyrm; SEQ ID NO:2), Eb (rsvraqrhtdmpktqkyqppstnknt ksqrrkgwpkthpggeqkegteaslqirgkkkeqrreigsrnaecrgkkgk; SEQ ID NO:3), and Ec (rsvraqrhtdm pktqkyqppstnkntksqrrkgstfeerk; SEQ ID NO:4) peptides and range from 35 to 87 amino acids in length and encompass a common sequence region at the N-terminus and a variable sequence region at the C-terminus. For example, the wild-type open reading frame for the IGF-1-Ea encodes a polypeptide of 105 amino acids (gpetlcgaelvdalqfvcgdrgfyfnkptgygsssrrapqtgivde ccfrscdlrrlemycaplkpaksa rsvraqrhtdmpktqkevhlknasrgsagnknyrm; SEQ ID NO:5). In physiological expression, the E-peptides are cleaved off of the precursor by endogenous proteases to yield the mature 70 amino acid IGF-1 known to be bioactive. In certain contexts, one to three of the N-terminal amino acids of IGF-1 are known to be cleaved under physiological conditions, yielding active IGF-1 having between 67-70 amino acids. IGF-2 gene expression and processing is characterized by similar attributes except that only one E-peptide (rdvstpptvlpdnfprypvgkffqydtwkqstqrlrrglpallrarrghvlakeleafreakrhrplialptqdpahggappemasnrk; SEQ ID NO:6) for human IGF-2 has been identified for the 156 amino acid precursor (ayrpsetlcggelvdtlqfvcgdrgfyfsrpasrvsrrsrgiveeccfrscdlalletycatpakserdvstpptvlpdnfprypvgkffqy dtwkqstqrlrrglpallrarrghvlakeleafreakrhrplialptqdpahggappemasnrk; SEQ ID NO:7). Both IGF-1 and IGF-2 appear to be poor drug candidates, since these proteins are quickly degraded by endogenous proteases in the serum of patients. One strategy that has been contemplated is to stabilize IGF-1 as a drug by forming a complex with one of its binding proteins.